1. Field of the Invention
This invention relates to processes for hydrolyzing starches and more particularly, to such processes especially adapted to provide substrate sugars for the fermentation to ethanol.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth (see, for example, U.S. Pat. No. 3,236,740 and the booklet "Industrial Alcohol by Continuous Fermentation and Vacuum Distillation With Low Energy Consumption", of Chemapec, Inc., Woodbury, N.Y.). For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy and raw material requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure and waste incurred in carrying out any of the aforesaid discrete operations involved in the manufacture of ethanol from vegetative sources.
Processes for the acid hydrolysis of starch to provide fermentable saccharides are known (viz, the acid starch hydrolysis processes described in U.S. Pat. Nos. 2,203,325; 2,210,659; 2,359,763; 2,393,095; 2,395,907; 2,565,404; 2,946,706; 2,954,304; 2,989,425; 3,169,083; 3,200,012; 3,236,687; 3,313,654; 3,446,664; 3,484,287; 3,607,395; and, 4,137,094). It is also known from U.S. Pat. No. 2,529,131 to subject still bottoms, or "vinasse", containing unfermented sugars to acid hydrolysis to convert said sugars to fermentable substrate. In all of the aforesaid acid hydrolysis processes, prior to conversion of the aqueous fermentable hydrolysate to ethanol employing yeast, the hydrolyzing acid must be neutralized with base. This results in the presence of relatively substantial quantities of salt in the fermentation feed, a condition which is decidedly disadvantageous for optimum ethanol production. And since the salts serve no useful function for any of the discrete operations involved in the conversion of starch to ethanol, their presence as by-products in the sugar liquor merely represents a wasteful consumption of raw materials, i.e., acid and neutralizing base.
Accordingly, there has heretofore existed a need for a process for hydrolyzing an aqueous starch slurry to a solution of fermentable sugar starch at rapid and high levels of conversion while dispensing with or minimizing the need to neutralize acid present in the sugar solution.
In addition to the foregoing disadvantage, while these and similar processes are relatively adaptable to the hydrolysis of the finely divided, relatively pure starch derived from conventional processes of wet milling cereal grains, their application to the starch-containing fractions obtained from processes of dry milling cereal grains as currently practiced would be uneconomically wasteful of the protein and edible oil associated with these fractions which in the case of corn and milo, is especially significant. Wet milling processes typically remove all but an insignificant amount of non-starch materials, i.e., protein, cellulosic fiber and oil, from the starch component of the grain, the non-starch materials finding valuable application in their own right as animal feeds and feed supplements. However, from the standpoint of producing starch for conversion to sugar, the sugar to dilute ethanol and the dilute ethanol to essentially anhydrous ethanol, conventional wet-milling processes are undesirable because of the need to ultimately remove the large amounts of process water involved.
Where, as in the case of low cost industrial ethanol, a minimal use of energy is necessary to achieve an economically viable process, a relatively energy and capital intensive process such as one based on wet-milled corn starch as the starting material can be disadvantageous. For this reason, the hydrolytic conversion of starch derived from any of the known and conventional dry milling processes is especially desirable in an industrial scale anhydrous ethanol program since these processes employ no added water beyond the moisture which is already naturally present in the grain. Thus, for example, in a typical dry corn milling process, the kernels are broken by impact and the resulting fractions made up of grits and fine feed which contain the bulk of the starch and significant quantities of oil, protein and cellulosic fiber, germ which contains most of the oil content of the kernels, and hulls which contain the major portion of the fiber, are separated employing degerminators, sifters, aspirators and gravity separators. A typical dry corn milling product analysis (pounds per bushel) is as follows:
______________________________________ DRY DEGERMINATION PRODUCTS ANALYSIS ON YELLOW CORN #2, LB/100 LB (DRY BASIS) CORN GERM BRAN GRITS ______________________________________ Yield, % 100 10 8 82 Ash 1.63 0.45 0.32 0.86 Fat (Oil) 4.30 2.20 0.36 1.74 Protein 9.00 1.36 0.56 7.08 Fiber 2.56 0.43 1.20 0.93 Starch 72.00 2.80 2.40 66.80 Other (Nitrogen Free 10.51 2.76 3.16 4.59 Extract) Total 100.00 10.00 8.00 82.00 ______________________________________
As this analysis indicates, the grits contain 92.8% of the starch, 78.7% of the protein and 40.5% of the oil of the whole corn kernels. Direct complete hydrolysis of the grits would therefore make these substantial amounts of protein and oil unavailable for use as comestibles.
Accordingly, there has heretofore existed a need for a process for converting starch derived from dry milled cereal grains to fermentable sugars while recovering substantially all of the protein and oil content of the starch component of the dry milled grain prior to the complete hydrolysis of the starch. The term "cereal grain" as used herein is to be understood in its commonly used sense and is inclusive of all varieties of corn (maize), milo, wheat, rice, and the like.